HomeMy WebLinkAboutDrainage Reports - 04/18/2009,
❑°i❑° MERRICK'
❑❑❑ BUILDING QUALITY SOLUTIONS�p rCuY:l®j'r.
Merrick & Company MR �i,7$ l�1sELRN-9
2450 S. Peoria Street
Aurora, CO 80014-5475
Phone: 303-751.0741 / Fax: 303-751-2581
www.merrick.com
March 9, 2009
Merrick Proj. #03015950
Mr. Glen Schlueter
City of Fort Collins
281 North College Ave.
Fort Collins, CO 80522-0580 Q_��?� �
RE: Chick-fil-A at the Cottonwood hopping Center — Commercial Drainage
Dear Mr. Schlueter:
The intent of this letter is t6 address drainage patterns with regard to the proposed Chick-fil-A
development at the Cottonwood Shopping Center located at the southwest corner of Horsetooth Rd. and
College Ave. The site is located in Lot 1, Block 1 replat of Lots 1, 2, 3, and 4 of the Cregger Plaza
Subdivision, northeast quarter of Section 35, Township 7 North, Range 69 West of the 6's Principal
Meridian in the city of Fort Collins, Colorado. The Chick-fil-A site consists of 14,701 square feet or 0.34
acres.
Currently, the existing site is occupied by a vacant restaurant building with outdoor seating patios,
sidewalks, landscaping and utilities. The ground is characterized as developed landscaping and irrigated
short grasses. The existing Larimer County No. 2 Canal also flows on the site along the northeastern
edge. Existing drainage patterns consist of overland sheet flows east and south either directly into the
existing canal or traveling offsite over the existing adjoining parking/drive areas within the shopping
center and into the Canal via a concrete Swale located southeast of the site. The existing imperviousness
of the site is roughly 10,200 square feet, or approximately 70% of the site.
The proposed development will replace the existing building with a free-standing quick -service restaurant
with drive-thru service and improve existing landscaping on the site. Disturbed areas include
approximately 13,240 square feet or 90% of the site, however the existing No. 2 Canal will be minimally
impacted. Erosion control measures will be implemented to minimize sedimentation and protect the
existing Canal. The proposed site includes drainage improvements with area inlets that collect runoff at
the southwest and southeast corners of the site. Runoff is conveyed via storm sewer pipe and discharged
to the existing No. 2 Canal. A portion of the south bank of the No. 2 Canal approximately 60 square feet
in size will be disturbed for the construction of the outfall.
Water quality is also provided with a "snout structure" built in to the farthest downstream area inlet. This
structure is a hood device installed inside an inlet at the outlet pipe to prevent oil and debris from exiting
and passing through the remainder of the storm sewer system. (Please see attached cut sheet.) Chick-fil-
A will be responsible for the oil/debris removal and maintenance of this structure.
An Employee -Owned Corporation
.................................................................................................................................................................................................. .
■ENGINEERING ■ARCHITECTURE ■DESIGN -BUILD ■ GEOSPATIAL SOLUTIONS ■SURVEYING
The undisturbed portion of the site (1,461 square feet or 10% of the site) includes the existing slope and
banks of the No. 2 Canal which will retain its historic flow patterns. Upon completion, the proposed site
will consist of 10,360 square feet impervious area, or approximately 70.5% of the site. The total net
impervious area of our site will be increased by approximately 160 square feet or 0.5% to a total site
imperviousness of 70.5%.
In summary, the proposed Chick-fil-A development will improve the existing drainage patterns of the site
by providing area inlets and water quality. The site's imperviousness area will increase by approximately
160 square feet or 0.5%. The existing Larimer County No. 2 Canal will be minimally affected by the
construction of the proposed storm outfall, and erosion control methods will be implemented on the site to
protect the Canal as well as reduce sedimentation of the site.
Thank you for your time regarding this matter
Very truly yours,
MERRICK & COMPANY
Todd Hepworth
Project Engineer
itact us.
EX SANTARY SERER MNN
EX SAN MN 7
EX SAN MH INV IN (W) = 27.60 ".•` - JT(♦-'�--(•
INV IN (W) = 2132 INV OUT (EJ = 22,51
WV OUT (E) = 23 JO � v
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b I 'f�`j'�. MSIONE/STUCCO - 128 SEATS /!
..___._._-.PROPOSED TRASH ENaoANEE I9 I � '- .: PROPOSED TYPE 11 Oleo S
NV OUT (S)kkk
A
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NFROX 25 PROPOSED AREA NIET - ✓w� -•
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NY QR . 2/.71
1 s�ti R 31.6! R .YLI< R J0. J6\ �' f14J002 R 29.66
X X x x.
EX CURB h GUTTER (Tro) ' • '•,• A 1 ) :I
I� \ p 1 '+.:::. PROPOSED ABPA III -
1, .SHOUT SIRIICIIAE
I EX' EDGE CY B1ROING SEf OETA6 THIS SHEEP
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20' 0 10' 20' w'
GRAPHIC SCALE
HYDROLOGICAL DATA
DESIGN POINT
CONTRIBUTING BASIN
WATERSHED AREA
RUNOFF COEFFICIENT
10 YR VOLUME
100 YR VOLUME
C10
I C100
(AC)
(CFS)
(CFS)
1
A
0.18
0.86
0.91
1.57
1.67
2
B
0.25
0.69
0.75
1.17
1.80
3
C
0.03
0.25
0.31
0.08
0.10
4
0
0.01
0.69
0.75
0.05
0.05
5
E
0.05
0.79
0.84
0.40
0.43
EX GROY/ND
SI1W VARIES
PROPOSED 12• PVC
O I.DD%
EX CLWC RING WALL
EX 7RAMC POLE '
EX UNOERGROUNO UTILITY ✓AULT
^JO\\
EX CCWC Avw sw
\EX PSCO
r{t
J/EAT
G1 '
tt
\I'' �
POAER dWCRLW 80)
ETAII WALL
IRAL PLANS
SAN MY �WV W (.ff) = 2.113
EX I' .90EN'A[K I
W Q
\WVW (0) 24.61
XWV ovr (N) - 2289\
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EX Nl' S4N/TARY j
EASFAIfNT "-
SWAX
! BCYLARRO.
UNkNGNN
D(B' SIDEWALK I
k,k- EX NAAWAILS I
Ex L r PLTE R7 RuAW
' r=V (S) JJ4
WV (X/T (NWJ _ 2J25
}
EX C
URB d! LV77ER
� I
1 WLET
WV . 5025..24
II
- 6.45 � ccl 2 CANAL
SED GRADE 7 ('� 11
CONC WINGWALL
EX PARR SURFACE
= 26.00
WSE = 24.65
PER MERRICK SURVEY
11/17/08
.�O OUTFALL DETAIL
NOT TO SCALE
ORATE
'G
L.
/ ANTI -SIPHON EEVICE
e •
ILL'SNOU
OIL M DEBRIS STOP
° OIL & FLOATABLE DEBRIS
A ON SURrACE
CANNOT E IT PIPE
4EM=J1
OUTFLOW
INFLOW
[
T
A .:
SUSPENDED sales ° c°
SETTLE ON BOTT
a
a
d
BMP, INC.
53 ATT. ARCHER ROAD, LYME, CT. M71
(BOO) SOFB008 FAX' (860)4 195
mmHml ra aABr<
SNOUT INLINE 1 09/17/99 NONE
BtauT USAATE2A81 NT WI) Apa1T@IN. PATENTS PENa1 D0
INSTALLATION �SAN
C-I
C-IN
INLET WITH WATER QUALITY SNOUT
NOT TO SCALE
A
B
'�
B
NO.. 12 Did[ ar4 From OV
mcfew ir 9p .'d �AL
A
PLAN VIEW
In•-
Ir SA >Ax' -
n� Tin -^--•�•�
"sal I '4 V.'
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:.4� •�,••
SECTION A -A
SECTION B-B
5200 Buffington Rd.
Atlanta, Georgia
30349-2998
Dote By
Date By
Date By
on,
STORE
CHICK-FIL-A
FSU S06A
COLLEGE AVE.
HORSETOOTH RD.
3620 S. MASON ST.
FT. COLLINS, CO 80525
SHEET TITLE
DRAINAGE
EXHIBIT
) No. :4391.525F
)re :2346
te :12/17/0e
)wn By : BTP
ecked By: SA
et
CONFIGURATION DETAIL
TYPICAL INSTALLATION
n m.r erm c�syON
'ER
BLE
POP
..... `ET PIPE (HIDDEN)
FRONT VIEW
SNOUT OIL -WATER -DEBRIS SEPARATOR
SIDE VIEW
NOTES:
1. ALL HOODS AND TRAPS FOR CATCH BASINS AND WATER QUALITY STRUCTURES SHALL BE
AS MANUFACTURED BY:
BEST MANAGEMENT PRODUCTS, INC.
53 MT. ARCHER RD.
LYME. CT 06371
(860) 434-0277, (860) 434-3195 FAX
TOLL FREE: (800) 504-8008 OR (888) 354-7585
WEB SITE: www.bestrnp.com
OR PRE -APPROVED EQUAL
2. ALL HOODS SHALL BE CONSTRUCTED OF A GLASS REINFORCED RESIN COMPOSITE WITH
ISO GEL COAT EXTERIOR FINISH WITH A MINIMUM 0.125' LAMINATE THICKNESS.
3. ALL HOODS SHALL BE EQUIPPED WITH A WATERTIGHT ACCESS PORT, A MOUNTING FLANGE,
AND AN ANTI -SIPHON VENT AS DRAWN. (SEE CONFIGURATION DETAIL)
4. THE SIZE AND POSITION OF THE HOOD SHALL BE DETERMINED BY OUTLET PIPE SIZE AS
PER MANUFACTURER'S RECOMMENDATION.
5. THE BOTTOM OF THE HOOD SHALL EXTEND DOWNWARD A DISTANCE EQUAL TO 12 THE
OUTLET PIPE DIAMETER WITH A MINIMUM DISTANCE OF 6- FOR PIPES <12- I.D.
6. THE ANTI -SIPHON VENT SHALL EXTEND ABOVE HOOD BY MINIMUM OF WAND A MAXIMUM OF
24- ACCORDING TO STRUCTURE CONFIGURATION.
7. THE SURFACE OF THE STRUCTURE WHERE THE HOOD IS MOUNTED SHALL BE FINISHED
SMOOTH AND FREE OF LOOSE MATERIAL.
8. THE HOOD SHALL BE SECURELY ATTACHED TO STRUCTURE WALL WITH 3/8' STAINLESS
STEEL BOLTS AND OIL -RESISTANT GASKET AS SUPPLIED BY MANUFACTURER. (SEE
INSTALLATION DETAIL)
9. INSTALLATION INSTRUCTIONS SHALL BE FURNISHED WITH MANUFACTURER SUPPLIED
INSTALLATION KIT.
INSTALLATION KIT SHALL INCLUDE:
A. INSTALLATION INSTRUCTIONS
B. PVC ANTI -SIPHON VENT PIPE AND ADAPTER
C. OIL -RESISTANT CRUSHED CELL FOAM GASKET WITH PSA BACKING
D. 3/8- STAINLESS STEEL BOLTS
E. ANCHOR SHIELDS
US Patent # 6126817
P°�DEVICE
<IS
,RIS
A\
8
°
a
SEE NOTE'
.. SOLIDS SETTLE ON °•
° BOTTOM
-NOTE- SUMP DEPTH OF 3(- MIN. FOR < OR=12- DIAM.
OUTLET. FOR OUTLETS >OR= 15-, DEPTH = 2.5-3X DIAM.
INSTALLATION DETAIL
DETAIL B
FOAM GASKET W/
PSA BACKING
° (TRIM TO LENGTH)
ANCHOR W/BOLT MOUNT!
FLANGE
(SEE DETAIL A)
D INSTALLATION NOTE,
eml
POSITION HOOD SUCH THAT
T_ BOTTOM FLANGE IS A
IIZI 12 D DISTANCE OF 12 OUTLET
—L PIPE DIAMETER (MIN.)
BELOW THE PIPE INVERT.
GASKET MINUMUM DISTANCE FOR
COMPRESSED PIPES < 12- I.D. IS 6
BETWEEN HOOD
+ AND STRUCTURE DETAIL
(SEE DETAIL B)
• DRILLED ANCHOR
HOLE SHIELD S AINLESS
BOLT
EXPANSION CONE
(NARROW END OUT)
HOOD SPECIFICATION FOR
CATCH BASINS AND
WATER QUALITY STRUCTURES
DESCRIPTION
OIL- DEBRIS HOOD
SPECIFICATION AND
DATE
(09/08/001
I SCALE
NONE
DRAWING NUMBER
SP-S N
INSTALLATION
(TYPICAL)
1" PVC ANTI -SIPHON
PIPE ADAPTER 6.50"
25.00"
lextil Y
Is
20.00"
24.00"
FRONT
NW.1wI
REMOVABLE WATERTIGHT
018.00" ACCESS PORT, 6" OPENING
/ 022.00" T
I
16.00"
R10.00"
2.00"
27.00"
10.00"
12.00"
SIDE
US PATENT #6126817ADDmONAI. PATENTS PENDING
BMP, INC.
53 MT. ARCHER ROAD, LYME, CT. 06371
(800) 504-8008 FAX. • (860)434-3195
DESCRIPTION DATE I SCALE
18F SNOUT OIL 09/14/99 NONE
& DEBRIS STOP DRAWING NUMBER
f2F
MERRICK COMPANY
Chick fil-A at Cottonwood Shopping Center
Project #03015950
Runoff Coefficient Calculation
Storm Return
Period
Land Use
Runoff
Coefficient*
Correction
Factor t
Computed Runoff
Coefficient C
5
Road/sidewalk Pavement
0.95
1.00
0.95
5
Roof
0.95
1.00
0.95
5
Lawn, Heavy Soil 2 to 7%
0.25
1.00
0.25
10
Road/sidewalk Pavement
0.95
1.00
0.95
10
Roof
0.95
1.00
0.95
10
Lawn, Heavy Soil 2 to 7%
0.25 _
1.00
0.25
100
Road/sidewalk Pavement
0.95
1.25
1.00
100
Roof
0.95
1.25
1.00
100
Lawn, Heavy Soil 2 to 7%
0.25
1.25
0.31
* From Table 3-3
t From Table 3-4
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Chick fil-A at Cottonwood Shopping Center
Project #03015950
Runoff Volume Calculation
Basin
Tc (min)
I (in/hr)
From Figure 3-1
Calculated Runoff
Coefficient, C10
Calculated Runoff
Coefficient, CI00
Area (ac)
Q10
(cfs)
Q100 (cfs)
A
5.00
9.9
0.89
0.94
0.18
1.62
1.71
B
5.00
9.9
0.86
0.91
0.20
1.72
1.83
C
5.00
9.9
0.33
0.39
0.03
0.10
0.12
D
5.00
9.9
0.69
0.75
0.01
0.05
0.05
E
5.00
9.9
0.79
0.84
0.05
0.40
0.43
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DRAINAGE CRITERIA MANUAL /3 A5 t / " -5
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VELOCITY IN FEET P R SECOND
21�
ESTIMATE OF AVERAGE FLOW VELOCITY FOR
USE WITH THE RATIONAL FORMULA.
10 20
' MOST FREQUENTLY OCCURRING "UNDEVELOPED"
LAND SURFACES IN THE DENVER REGION.
REFERENCE:: "Urban Hydrology For Small Watersheds"
Technical Release No. 55, USDA, SCS Jan. 1975.
5-1-84
URBAN DRAINAGE & FLOOD CONTROL DISTRICT
DRAINAGE CRITERIA MANUAL f
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VELOCITY IN FEET PER SECOND
ESTIMATE OF AVERAGE FLOW VELOCITY FOR
USE WITH THE RATIONAL FORMULA.
10 20
' MOST FREQUENTLY OCCURRING "UNDEVELOPED"
LAND SURFACES IN THE DENVER REGION.
REFERENCE:: "Urban Hydrology For Small Watersheds'
Technical Release No. 55, USDA, SCS Jan. 1975.
5-1-84
URBAN DRAINAGE & FLOOD CONTROL DISTRICT
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Figure 3-3 ESTIMATE OF AVERAGE FLOW VELOCITY FOR
USE WITH THE RATIONAL FORMULA.
MOST FREQUENTLY OCCURRING "UNDEVELOPED"
LAND SURFACES IN THE DENVER REGION.
REFERENCE:: "Urban Hydrology For Small Watersheds'
Technical Release No. 55, USDA, SCS Jan. 1975.
5-1-84
URBAN DRAINAGE & FLOOD CONTROL DISTRICT
Intensity (in/hr)
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13130 BUILOINB OUALITV BOLUTIONB Contract
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❑❑io MERRICK& Engineering Calculation Sheet Date sheet 2 of Z
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Calculation No.
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�p� lcwla�ions
... •• �J iJ LV LI LL L3 L4 Lb [b 27 28 29
M F. llL/Rw BN2
Friction Method
Solve For
Storm Sewer Outfall to Canal
Manning Formula
Normal Depth
iP
In tit"Datas�
Roughness Coefficient
0.010
Channel Slope
0.01000 ft/ft
Diameter
1.00 ft
Discharge
4.05 ft3/s
11eSUItS:._ ;+ ni "2" �.:'�§i � �'"� a�� �a`� �i a x ��, =•+'.I�'-'r4 Y3J P., ....xK
Normal Depth
Flow Area
Wetted Perimeter
Top Width
Critical Depth
Percent Full
Critical Slope
Velocity
Velocity Head
Specific Energy
Froude Number
Maximum Discharge
Discharge Full
Slope Full
Flow Type SuperCritical
0.72 ft
0.61 ft'
2.04 ft
0.89 ft
0.85 ft
72.4 %
0.00718 Wit
6.65 ft/s
0.69 ft
1.41 ft
1.42
4.98 ft3/s
4.63 ft3/s
0.00765 ft/ft
-� — —
�GVF.jhput Data i .r: tag'" U , �'; ` " �, , �, ' j.„X , `;A -9
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Length 0.00 ft
Number Of Steps 0
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Upstream Depth
0.00
ft
Profile Description
Profile Headloss
10.00
ft
Average End Depth Over Rise
0.00
Normal Depth Over Rise
72.43
%
Downstream Velocity
Infinity
ft/s
Upstream Velocity
Infinity
fUs
Bentley Systems, Inc. Haestad Methods Solution Center Bentley FlowMaster [08.01.071.00]
1/7/2009 8:35:58 AM 27 Siemons Company Drive Suite 200 W Watertown, CT 06795 USA +1-203-755.1666 Page 1 of 2
Storm Sewer Outfall to Canal
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Critical Depth
Channel Slope
Critical Slope
0.72 ft
0.85 ft
0.01000 ft/ft
0.00718 ft/ft
Bentley Systems, Inc. Haestad Methods Solution Center Bentley FlowMaster [08.01.071.001
177/2009 8:35:58 AM 27 Siemons Company Drive Suite 200 W Watertown, CT 06795 USA +1-203-755-1666 Page 2 of 2
DRAINAGE CRITERIA MANUAL (V. 2)
HYDRAULIC STRUCTURES
30
25
aP
1 20
E
15
n
01
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0
5
1
*Grouted BIB Boulders
-- - - - - - - - - -- - - -; --- - - -- --
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- - ,-
2
3
4
5
6 7 8
Storm
Sewer
Diameter,
D, or
Height, H, in ft.
Figure HS-20c—Low Tailwater Riprap Basins for Storm Sewer Pipe Outlets—
Riprap Selection Chart for Low Tailwater Basin at Pipe Outlet
(Stevens and Urbonas1996)
Rev. 2008-04 HS-83
Urban Drainage & Flood Control District
O
4
E
7
E
9
11
1'
1:
1.
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oio MERRICK° Engineering Calculation Sheet Date (2-1)O8 Sheet ► of
❑❑❑ BUILDING SUALITV SOLUTIONS Contract
Calculation No.
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Revision
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31
10 E ' 1
ARE
SINGLE PIPE
HEADWALL INSTALLATIONS
TYPE OF PIPE HEADWALL DIMENSIONS
D- Pipe diameter
H= D+30'
D W-- 30 t IB"
T= 0.4H (nearest inch)
B= 1.50 (30"minimum)
CIRCULAR L= W+B
NOTE: Volume occupied by pipe has been deducted.
TYPE OF PIPE
QUANTITIES FOR ONE CONCRETE HEADWALL
F.
Lam,.
COW
DOUBLE PIPE
HEADWALL DIMENSIONS
SPAN i -+y D= Equivaiacircular diamm eter
4, Hz Rise +3e
3D +
T
=Q4H(neneauest inch)
B= 1.5 D (30 minimum)
ARCH L= W t B
(Cubic Yards)
R[P
1-3
1.8
1 1.7
2.3
2-331
2.9
3.9
3.1
e 4
CSP 1 - 0.9
1.3
t.4
1 1.9
1 1.8
1 2.4
1 2.4
1 34
1 3.2
1 44
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DBL
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GENERAL NOTES:
1. REVISED COON - U-601-12
HEADWALLS FOR PIPES
(15" TO 48" DIAMETER)
CITY OF FORT COLLINS7 COLORADO
ENGINEERING SERVICES UNIT
APPROVED BY:
DATE: -3 //_ Ard
REVISIONS:
D-171
3.1.6 Runoff Coefficients
The runoff coefficients to be used with the Rational Method referred to in
Section 3.2 "Analysis Methodology" can be determined based on zoning
classifications if the character of the surface is unknown. However, the final
drainage study must calculate a composite coefficient using Table 3-3. Table 3-2
lists the runoff coefficients for the various types of zoning along with the
zoning definitions. Table 3-3 lists coefficients for the different kinds of
surfaces. Since the Land Development Guidance System for Fort Collins allows
land development to occur which may vary the zoning requirements and produce
runoff coefficients different from those specified in Table 3-2, the runoff
coefficients should not be based solely on the zoning classifications. The
runoff coefficient used for design should be based on the actual conditions of
the proposed development.
The Composite Runoff Coefficient shall be calculated using the following formula:
n
C = (CiAi) l Al
i=1
--..Where C = Composite Runoff Coefficient
C1= Runoff Coefficient for specific area Al
A,= Areas of surface with runoff coefficient of C1
n = Number of different surfaces to be considered
At= Total area over which C is applicable; the sum of all Ai's is equal
to At
Table 3-2
RATIONAL METHOD XMR STORM RUNOFF' COEFFICIENTS FOR ZONING CLASSIFICATIONS
Busiaes BP,BL..................................... 0.85
Business: HB,C................................... 0 5
Industrial: Z P.............................. 0.85
Industrial: IG... ..... 0.95
Residential: RE,RLP.. 0.45
Residential: RL,ML,RP.. 0.50
Residential: RLM,RMP...... ................. 0.60
Residential: RM,MM........ 0.65
Residential: RH....... ...... 0.70
Parks, Cemeteries ............. ............. 0,25
Playgrounds .. .......... 0.35
Railroad Yar eas ...... 0.40
Unimprov eas ..... 0.20
Zoning Definitions
_. R-E Estate Residential District - a low density residential area primarily in
outlying areas with a minimum lot area of 9,000 square feet.
R-L Low Density Residential District - low density residential areas located
throughout the City with a minimum lot area of 6,000 square feet.
R-M Medium Density Residential District - both low and medium density
residential areas with a minimum lot area of 6,000 square feet for one -
family or two-family dwellings and 9,000 square feet for a multiple
family dwelling.
R-H High Density Residential District - high density residential areas with a
minimum lot area of 6,000 square feet for one -family or two-family
dwellings, 9,000 square feet for a multiple family dwelling, and 12,000
square feet for other specified uses.
R-P Planned Residential District - designation of areas planned as a unit
(PUD) to provide a variation in use and building placements with a
minimum lot area of 6,000 square feet.
May 1984
Revised January 1997
Design Criteria
3-3
Table 3-3
RATIONAL METHOD RUNOFF COEFFICIENTS FOR COMPOSITE ANALYSIS
Character of Surface Runoff Coefficient
Streets, Parking Lots, Drives:
Asphalt...................................... 0.95
Concrete ..................................... 0.95
Gravel ....................................... 0.50
Roofs.......................................... 0.95
Lawns, Sandy Soil:
Flat<2%..................................... 0.10
Average 2 to 7%.............................. 0.15
Steep>78.................................... 0.20
Lawns, Heavy Soil:
Flat<2%..................................... 0.20
Average 2 to 7%.............................. 0.25
Steep>78..................................... 0.35
3.1.7 Time of Concentration
In order to use the Rainfall Intensity Duration Curve, the time of
concentration must be known. The time of concentration, T., represents the
time for water to flow from the most remote part of the drainage basin under
consideration to the design point under consideration. The time of
concentration can be represented by the following equation.
Tc = t., + t,
Where:
Tc = Time of Concentration, minutes
t„ = overland flow time, minutes
tt= travel time in the gutter, swale, or storm sewer, minutes
The overland flow time, t,,, ,can be determined either by the following equation
or the "Overland Time of Flow Curves" from the Urban Storm Drainage Criteria
Manual, included in this report (See Figure 3-2).
1.87(1.1-af)DU2
Tw= SI/3
Where: T„ = Overland Flow Time of Concentration, minutes
S = Slope, %
C = Rational Method Runoff Coefficient
D = Length of Overland Flow, feet'(500' maximum)
Cc = Frequency Adjustment Factor
The travel time, t,, in the gutter, Swale, or storm sewer can be estimated with
the help of Figure 3-3.
3.1.8 Adjustment for Infrequent Storms
The preceding variables are based on the initial storm, that is, the two to ten
year storms. For storms with higher intensities an adjustment of the runoff
coefficient is required because of the lessening amount of infiltration,
depression retention, and other losses that have a proportionally smaller
effect on storm runoff.
These frequency adjustment factors are found in Table 3-4.
May 1984
Revised January 1997
Design Criteria
3-5
Table 3-4
RATIONAL METHOD FREQUENCY ADJUSTMENT FACTORS
Storm Return Period Frequency Factor
(years) Cf
2 to 10 1.00
11 to 25 1.10
26 to 50 1.20
51 to 100 1.25
Note: The product of C times Ct shall not exceed 1.00
3.2 Analysis Methodology
The methods presented in this section for use in the determination of runoff at
specific design points in the drainage system are currently under review by the
Stormwater Utility. Until detailed criteria for hydrologic modeling are developed, the
accepted methods for hydrologic analysis are (1) the Rational Method and (2) UDSWM2-
PC. The Stormwater Utility shall determine circumstances requiring computer modeling
with UDSWM2-PC. Early contact with the Stormwater Utility is encouraged for the
determination of the appropriate method. Where applicable, drainage systems proposed
for construction should provide the minimum protection as determined by the methodology
so mentioned above.
3.2.1 Rational Method
The Rational Method is recommended only for sites less than 5 acres. The runoff
may be calculated by the Rational Method, which is essentially the following
equation:
Q = CCCIA
Where Q = Flow Quantity, cfs
A = Total Area of Basin, acres
C== Storm Frequency Adjustment Factor (See Section 3.1.8)
C = Runoff Coefficient (See Section 3.1.6) !
I = Rainfall Intensity, inches per hour (See Section 3.1.4)
3.2.2 UDSW42-PC
For circumstances requiring computer modeling, the design storm hydrographs
shall be determined using UDSWM2-PC. Basin and conveyance element parameters
shall be developed from the physical characteristics of the development. Refer
to the UDSWM2-PC User's Manual* for modeling methodology and development.
'Urban Drainage and Flood Control District, March 1985
3.2.2.1 Surface Storage and Infiltration
Table 3-5 gives those values for surface storage for pervious and
impervious surfaces. Table 3-6 gives the infiltration rates to be used
with UDSWM2-PC.
Table 3-5
VALUES FOR SURFACE STORAGE
(All Values in Inches)
(For Use with UDSWM2-PC)
Impervious Areas .................. .100
Pervious Areas .................... .300
May 1984
Revised January 1997
Design Criteria
3-6
Table 3-6
INFILTRATION RATES
(For Use With UDSWM2-PC)
Maximum Infiltration Rate......... .51 inch/hr
Minimum Infiltration Rate......... .50 inch/hr
Decay Rate ........................ .0018 inch/hr
3.2.2.2 Pervious - Impervious Area
In order to determine preliminary percentages of impervious land cover
for a given land use or zoning, Table 3-7 shall be utilized. The final
design shall be based on actual conditions of the development.
Table 3-7
LAND USE VERSUS PERCENT OF IMPERVIOUSNESS
(For Preliminary UDSWM2-PC Model Only)
Land Use or Zonin t Percent Pervious Percent Impervious
Business:
BG,BL,BP,HB,C,IL,IG,IP 10 90
Residential:
RH,RMP 40 60
RM.RP,MM 55 45
RM,RMP,RE,RLM,ML 60 40
Parks, Greenbelts, etc 90 10
en7See Table 3-2 for zoning definitions.
3.2.2.3 Resistance Factors
Table 3-8 contains the resistance factors to be used for pervious and
impervious areas.
Table 3-8
RESISTANCE FACTORS
(For Use With UDSWM2-PC)
Surface Resistance Factor
Pervious.......................................250
Impervious.....................................016
May 1984
Revised January 1997
Design Criteria
3-7
HYDRAULIC STRUCTURES
DRAINAGE CRITERIA MANUAL (V. 2)
Again, enter Figure HS-19a using the smaller d/D (or d/H) ratio to find the A/Af,11 ratio. Then,
Finally,
A = (A1Af,i )Af,, (HS-16c)
V = QIA (HS-16d)
In which for Equations 16a through 16d above:
Af.11 = cross -sectional area of the pipe (ft)
A = area of the design flow in the end of the pipe (ft)
n = Manning's n for the pipe full depth
0.11 = pipe full discharge at its slope (cfs)
R = hydraulic radius of the pipe flowing full, ft [Rfu11= D14 for circular pipes, Rf11= Af,,1/(2H + 2w) for
rectangular pipes, where D = diameter of a circular conduit, H = height of a rectangular
conduit, and w = width of a rectangular conduit (ft)]
So = longitudinal slope of the pipe (ft/ft)
V = design flow velocity at the pipe outlet (ft/sec)
Vfun = flow velocity of the pipe flowing full (ft/sec)
3.4.3.2 Riprap Size
For the design velocity, use Figure HS-20c to find the size and type of the riprap to use in the scour
protection basin downstream of the pipe outlet (i.e., B18, H, M or L). First, calculate the riprap sizing
design parameter, Pd, namely,
Pd = �V2 + gdl12 (HS-16e)
in which:
V = design flow velocity at pipe outlet (ft/sec)
g = acceleration due to gravity = 32.2 ft/sec2
d = design depth of flow at pipe outlet (ft)
HS-66 2008-04
Urban Drainage & Flood Control District
DRAINAGE CRITERIA MANUAL (V. 2)
HYDRAULIC STRUCTURES
Photograph HS-12—Upstream and downstream views of a low tailwater basin in Douglas
County protecting downstream wetland area. Burying and revegetation of the rock would
blend the structure better with the adjacent terrain.
When the riprap sizing design parameter indicates conditions that place the design above the Type H
riprap line in Figure HS-20, use B18, or larger, grouted boulders. An alternative to a grouted boulder or
loose riprap basin is to use the standard USBR Impact Basin VI or one of its modified versions, described
earlier in this Chapter of the Manual.
After the riprap size has been selected, the minimum thickness of the riprap layer, T, in feet, in the basin
is set at:
T=1.75D50 (HS-17)
in which:
D5n = the median size of the riprap (see Table HS-9.)
Table HS-9—Median (i.e., D50) Size of District's Riprap/Boulder
Riprap Type
D5u---Median Rock Size (inches)
L
9
M
12
H
18
B18
18 (minimum dimension of
grouted boulders)
3.4.3.3 Basin Length
The minimum length of the basin, L, in Figure HS-19, is defined as being the rg eater of the following:
for circular pipe: L = 4D or L = (Dlt/2(V)
J 2
(HS-18)
Rev. 2008-04
Urban Drainage & Flood Control District
HS-67
HYDRAULIC STRUCTURES
DRAINAGE CRITERIA MANUAL (V. 2)
1/2(2)
V
for rectangular pipe: L = 4H or L = (H)
in which:
L = basin length
H = height of rectangular conduit
V = design flow velocity at outlet
D = diameter of circular conduit
(HS-19)
3.4.3.4 Basin Width
The minimum width, W, of the basin downstream of the pipe's flared end section is set as follows:
for circular pipes: W = 4D (HS-20)
for rectangular pipe: W = w + 4H
in which,
W = basin width (Figure HS-19)
D = diameter of circular conduit
w = width of rectangular conduit
3.4.3.5 Other Design Requirements
All slopes in the pre -shaped riprapped basin are 2H to 1 V.
(HS-21)
Provide pipe joint fasteners and a structural concrete cutoff wall at the end of the flared end section for a
circular pipe or a headwall with wingwalls and a paved bottom between the walls, both with a cutoff wall
that extends down to a depth of:
D H
B=—+T or B=—+T
2 2
in which,
B = cutoff wall depth
D = diameter of circular conduit
T= Equation HS-17
The riprap must be extended up the outlet embankment's slope to the mid -pipe level.
(HS-22)
HS-68 2008-04
Urban Drainage & Flood Control District
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VELOCITY IN FEET PER SECOND
Figure 3-3 ESTIMATE OF AVERAGE FLOW VELOCITY FOR
USE WITH THE RATIONAL FORMULA.
MOST FREQUENTLY OCCURRING "UNDEVELOPED"
LAND SURFACES IN THE DENVER REGION.
REFERENCE:: "Urban Hydrology For Small Watersheds'
Technical Release No. 55, USDA, SCS Jan. 1975.
5-1-84
URBAN DRAINAGE & FLOOD CONTROL DISTRICT
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